Big 3 question about 2001 cadillac and wire questions

Im planning on doing the big 3 upgrade on a 2001 cadillac devielle but found out the battery is under the rear seat thus the alt in front. What i wanted to clear up was if i would just run a longer fused wire to under rear seat where the batt is located and the run batt + and then to alt + to get one of the big 3, the 2nd one being just the batt neg- to the chassis under the seat as well, and the last wire that requires engine ground to chassis under the hood in the front? I think it should be like this but this is one complicated car. Also, quick question, i've been reading around forums and saw people talk about several runs of wire to amp (i.e. 2 runs, 4 runs, ect), what exactly is this? Is it like just taking two wires from batt pos to run towards the back? And i see people stating that they run multiple grounds?? Can some of you guys please help me understand what all this means, it'd be much appreciated. Thanks